The presence of material optical elements (such as lenses, windows, prisms, and other elements used for beam handling and focusing) is a limitation on the fluence produced by a laser output. Material optical elements are often positioned after the final amplification stage of the laser and are exposed to the output fluence of the laser. If the output fluence exceeds the damage threshold of a material optical element, the element is damaged and performance of the laser is compromised. Also, the beam quality and, hence, focusability of a laser beam is limited by phenomena such as optical damage and nonlinear index growth in material optical elements. Limitations on output fluence and focusability imposed by material optical elements are detrimental to the use of lasers as drivers for inertial confinement fusion. The cost and commercial feasibility of laser-driven ICF is directly related to these constraints as well.
In laser-driven ICF systems, the laser is based on a solid state energy storage medium (e.g. Nd3+:glass). Energy is stored by using a low-energy input pulse to pump the solid state energy storage medium to a long-lived excited state and subsequently extracting energy in the form of a high-energy pulse with short duration. The pumping and extraction processes dictate the staging requirements for the amplification of the low energy input pulse to a high energy output pulse. The staging requirement often necessitate variable aperture sizes in different amplifier sections and/or regenerative designs with attendant complexity and beam quality limitations. In solid state energy storage lasers, intermediate spatial filtering may be utilized to suppress nonlinear growth in order to limit overall phase distortion and intensity fluctuations. The fundamental wavelength of many high energy solid state energy storage media is in the infrared (e.g. 1064 nm). Infrared wavelengths are suboptimal for the targets most commonly used in ICF. Wavelengths in the UV are preferred. As a result, additional material optical elements for frequency doubling or tripling are typically required when using solid state energy storage media in ICF applications. Such optics are also susceptible to damage when exposed to laser pulses having high power and impose further limitations on the cost and performance of ICF systems.